Self-biasing and self-sequencing of depletion-mode transistors

ABSTRACT

A transistor circuit includes a transistor having a gate terminal and first and second conduction terminals, a first circuit configured to convert an AC input signal of the transistor circuit to a gate bias voltage and to apply the gate bias voltage to the gate terminal of the transistor, a second circuit configured to convert the AC input signal of the transistor circuit to a control voltage, and a switching circuit configured to apply a first voltage to the first conduction terminal of the transistor in response to the control voltage.

BACKGROUND Technical Field

The disclosed technology relates to self-biasing of transistors and,more particularly, to self-biasing of depletion mode gallium nitridepower transistors.

Discussion of Related Art

Gallium nitride transistors are used for radio frequency poweramplifiers because they can operate at high temperatures and highvoltages. High power gallium nitride transistors are typically depletionmode devices, which are normally on at zero gate-source voltage. If avoltage is applied between the drain and the source when the gate-sourcevoltage is zero, a large, potentially destructive current may flowthrough the device. Accordingly, a negative gate bias voltage is appliedto the transistor before a voltage is applied between the drain and thesource, in order to limit current flow when the drain voltage isapplied. This implies sequencing of the voltages used to operatedepletion mode transistors.

A typical depletion mode transistor circuit is provided with a drainsupply voltage, a negative gate bias voltage and a triggering signalwhich enables or disables the gate bias, the drain supply voltage, orboth. An RF input signal is supplied to the gate of the transistor, andan amplified RF output signal is obtained at the output of the circuit.

The negative gate bias voltage typically requires use of a DC-DCconverter to convert a positive supply voltage to the negative gate biasvoltage. The DC-DC converter involves extra cost and extra circuit area.In addition, DC-DC converters can generate unwanted RF noise, which isproblematic in transmitter and receiver systems. Also, if a negativevoltage source is present in the system, a line has to be routed fromthe voltage source to the gate of the transistor, making the systemsusceptible to noise.

Another disadvantage of depletion mode transistors is the requirementfor the sequencing of the gate and drain voltages. The negative voltagemust be present at the gate before the drain voltage is applied. Thechannel of the depletion mode transistor is fully open with a floatingor grounded gate, and application of a drain voltage in this state maypermanently damage the device.

Accordingly, there is a need for improved transistor biasing circuits.

SUMMARY

The disclosed technology provides circuitry which uses an AC inputsignal, such as an RF input signal, to generate a gate bias voltage andto apply a drain voltage to the transistor. The disclosed technologyeliminates the need for a separate voltage source for the gate or aDC-DC converter. Because generating the gate bias voltage and switchingthe drain voltage are based on the input signal, sequencing of the gatebias voltage and the drain voltage can be achieved by selecting the timeconstants of the circuitry.

In accordance with embodiments, a transistor circuit comprises atransistor having a gate terminal and first and second conductionterminals, a first circuit configured to convert an AC input signal ofthe transistor circuit to a gate bias voltage and to apply the gate biasvoltage to the gate terminal of the transistor, a second circuitconfigured to convert the input signal of the transistor circuit to acontrol voltage, and a switching circuit configured to apply a firstvoltage to the first conduction terminal of the transistor in responseto the control voltage.

In some embodiments, the first and second circuits and the switchingcircuit are configured to apply the gate bias voltage to the gateterminal of the transistor before the first voltage is applied to thefirst conduction terminal of the transistor.

In some embodiments, the transistor comprises a depletion modetransistor. In further embodiments, the transistor comprises a galliumnitride depletion mode power transistor.

In some embodiments, the gate bias voltage is negative and the firstvoltage is a positive drain voltage.

In some embodiments, the first circuit comprises an RF coupler, arectifier and a voltage regulator.

In some embodiments, the second circuit comprises an RF coupler and arectifier.

In accordance with embodiments, a method is provided for operating atransistor having a gate terminal and first and second conductionterminals. The method comprises converting an AC input signal to a gatebias voltage and applying the gate bias voltage to the gate terminal ofthe transistor, converting the AC input signal to a control voltage, andapplying a first voltage to the first conduction terminal of thetransistor in response to the control voltage.

In accordance with embodiments, a transistor circuit comprises adepletion mode RF power transistor having a gate terminal, a drainterminal and a source terminal, a first circuit configured to convert aninput RF signal to a gate bias voltage and to apply the gate biasvoltage to the gate terminal of the transistor, a second circuitconfigured to convert the RF input signal to a control voltage, and aswitching circuit configured to apply a drain voltage to the drainterminal of the transistor in response to the control voltage, whereinthe first and second circuits and the switching circuit are configuredto apply the gate bias voltage to the gate terminal of the transistorbefore the drain voltage is applied to the drain terminal of thetransistor.

BRIEF DESCRIPTION OF DRAWINGS

The disclosed technology may be understood with reference to theaccompanying drawings, which are incorporated herein by reference and inwhich:

FIG. 1 is a schematic block diagram of a transistor circuit inaccordance with embodiments;

FIG. 2 is a timing diagram that illustrates sequencing of the gate biasvoltage and the drain voltage;

FIG. 3 is a schematic diagram of a transistor circuit in accordance withembodiments;

FIG. 4 is a schematic diagram of the first voltage conversion circuit inaccordance with embodiments;

FIG. 5 is a schematic diagram of the second voltage conversion circuitand the switching circuit in accordance with embodiments; and

FIG. 6 is a schematic block diagram of a transistor circuit inaccordance with further embodiments.

DETAILED DESCRIPTION

A schematic block diagram of a transistor circuit 10 in accordance withembodiments is shown in FIG. 1. The transistor circuit 10 includes atransistor 20 which receives an AC input signal, which may be an RFinput signal, through a DC blocking capacitor 22 and provides an RFoutput signal through a DC blocking capacitor 24. The transistor 20includes a gate terminal G, a drain terminal D and a source terminal Sand may, for example, be a gallium nitride RF power transistor whichoperates in the depletion mode. A depletion mode transistor is one whichis normally on at zero gate-source voltage with a drain voltage applied.In contrast, an enhancement mode transistor is normally off at zerogate-source voltage with a drain voltage applied. The transistor 20 isnot limited to a gallium nitride transistor and is not limited to adepletion mode transistor. The drain and source terminals may bereferred to as first and second conduction terminals.

The transistor 20 receives the RF input signal at the gate terminal Gand provides the RF output signal at the drain terminal D. The sourceterminal S of transistor 20 may be connected to a reference voltage,such as ground. The transistor circuit 10 further includes a firstvoltage conversion circuit 30, a second voltage conversion circuit 40and a switching circuit 50.

The first voltage conversion circuit 30 has an input coupled to the RFinput of the transistor circuit 10 and an output coupled to the gateterminal G of transistor 20. The first voltage conversion circuit 30samples the RF input signal and provides a gate bias voltage to the gateterminal G of transistor 20.

The second voltage conversion circuit 40 has an input coupled to the RFinput of the transistor circuit 10 and an output coupled to a controlinput of switching circuit 50. The second voltage conversion circuit 40samples the RF input signal and provides a control voltage to switchingcircuit 50.

The switching circuit 50 is coupled between a supply voltage and thedrain terminal D of transistor 20 and receives the control voltage fromthe output of second voltage conversion circuit 40. When the controlvoltage is inactive, in the absence of an RF input signal, the switchingcircuit 50 is turned off and the supply voltage is disconnected from thedrain terminal D of transistor 20. When the control voltage is active,in the presence of an RF input signal, the switching circuit 50 isturned on, and the supply voltage is applied to the drain terminal D oftransistor 20.

Operation of the transistor circuit 10 shown in FIG. 1 depends on thestate of the RF input signal. When the RF input signal is off, the firstvoltage conversion circuit 30, the second voltage conversion circuit 40and the switching circuit 40 and the switching circuit 50 aredeactivated. As a result, the gate bias voltage applied to gate terminalG is zero, and the switching circuit 50 is turned off, so that thesupply voltage is not applied to the drain terminal D of transistor 20.Thus, transistor 20 is in an off state.

Operation of transistor circuit 10 is described with reference to FIG.2. In FIG. 2, RF input signal 100, gate bias voltage 110 and drainvoltage 120 are plotted as a function of time. Initially, the RF inputsignal is zero, the gate bias voltage applied to gate terminal G is zeroand the drain voltage applied to drain terminal D is zero. At a time T0,the RF input signal is applied to the input of the transistor circuit10. At time T0, the gate bias voltage begins decreasing, and the drainvoltage applied to drain terminal D remains at zero volts. The gate biasvoltage decreases in response to sampling of the RF input signal asdescribed below. At a time T1, the gate bias voltage reaches asufficient negative value −VG for biasing of transistor 20 in an off orpartially off state. The drain voltage applied to drain terminal Dremains at zero at time T1. At a time T2 later than time T1, the secondvoltage conversion circuit 40 applies the control voltage to switchingcircuit 50. The switching circuit 50 applies the supply voltage to drainterminal D of transistor 20 in response to the control voltage.

FIG. 2 illustrates sequencing of the gate bias voltage and the drainvoltage. The gate bias voltage 110 is applied to the gate terminal G oftransistor 20 at time T1 before the drain voltage 120 is applied todrain terminal D of transistor 20 at time T2. Thus, the transistor 20 isbiased off before the drain voltage 120 is applied to the drainterminal, and damage to transistor 20 is prevented.

The sequencing illustrated in FIG. 2 and described above may be providedby the first voltage conversion circuit 30, the second voltageconversion circuit 40 and/or the switching circuit 50, since applicationof the gate bias voltage 110 and switching of the drain supply voltage120 are both initiated by the RF input signal. For example, the secondvoltage conversion circuit 40 may have a time constant that is longerthan a time constant of the first voltage conversion circuit 30 toensure that the gate bias voltage is applied to the gate terminal G oftransistor 20 before the control voltage is applied to switching circuit50. The delay between time T1 and time T2 should be sufficient to ensurethat transistor 20 is biased off or partially off and is not damaged bythe application of the supply voltage to the drain terminal D oftransistor 20.

In the example of FIG. 2, the gate bias voltage is negative, and thesupply voltage applied to the drain terminal D of transistor 20 ispositive. However, the gate bias voltage can be positive or negative,and the drain supply voltage can be positive or negative, depending onthe transistor type and the circuit configuration. Further, thetransistor 20 has been described as a depletion mode transistor.However, the transistor circuit 10 may be utilized with an enhancementmode transistor. In the case of an enhancement mode transistor,sequencing of the voltages applied to the transistor may not benecessary.

An embodiment of the transistor circuit 10 of FIG. 1 is shown in FIG. 3.Like elements in FIGS. 1 and 3 have the same reference numerals. In theembodiment of FIG. 3, the transistor 20 is an RF power GaN (galliumnitride) HEMT (high electron mobility transistor) and is a depletionmode transistor. A negative voltage generator corresponds to the firstvoltage conversion circuit 30 of FIG. 1, and a positive voltagegenerator corresponds to the second voltage conversion circuit 40 ofFIG. 1.

The transistor circuit 10 of FIG. 3 includes an input matching circuit210 coupled between DC blocking capacitor 22 and the gate terminal G oftransistor 20 and an output matching circuit 220 coupled between thedrain terminal D of transistor 20 and DC blocking capacitor 24. Aquarter wavelength bias line 222 is coupled between output matchingcircuit 220 and switching circuit 50. A capacitor 224 is coupled betweenquarter wavelength bias line 222 and ground.

In the embodiment of FIG. 3, the first voltage conversion circuit 30includes an RF coupler 230, a diode 232, a resistor 234, a capacitor236, a resistor 238, and a gate voltage regulator 240. The RF coupler230 samples the RF input signal and can be a directional coupler instripline or microstrip, for example. RF coupler 230 is coupled throughdiode 232 to a node 242. Diode 232 functions as a rectifier of thesampled RF input signal. Resistor 234 and capacitor 236 are connected inparallel between node 242 and ground. Resistor 238 is coupled betweennode 242 and the gate terminal G of transistor 20 via input matchingcircuit 210. The gate voltage regulator 240, which may be for example aZener diode, is coupled between node 242 and ground.

In operation, the RF coupler 230 samples the RF input signal, and thediode 232 rectifies the sampled RF input signal. The rectified RF inputsignal produces a negative voltage on node 242. The resistor 234 and thecapacitor 236 perform smoothing of the rectified voltage, and the gatevoltage regulator 240 establishes a fixed voltage on node 242. Thevoltage on node 242 is coupled through resistor 238 to the gate terminalG to provide a negative gate bias voltage in the embodiment of FIG. 3.

The second voltage conversion circuit 40 includes an RF coupler 250, adiode 252, a resistor 254 and a capacitor 256. The diode 252 isconnected between RF coupler 250 and a node 258. Diode 252 functions asa rectifier of the sampled RF input signal. The resistor 254 and thecapacitor 256 are connected in parallel between the node 258 and ground.

In operation, the RF coupler 250 samples the RF input signal, and thediode 252 rectifies the sampled RF input signal. The resistor 254 andthe capacitor 256 smooth the rectified voltage to produce a positivecontrol voltage on node 258. The control voltage on node 258 is suppliedto switching circuit 50 so as to control a switch state of switchingcircuit 50. The control voltage on node 258 has a sufficient magnitudeto activate the switching circuit 50 to an on switch state.

In the embodiment of FIG. 3, the switching circuit 50 includes atransistor 270, a resistor 272 and a transistor 274. In the embodimentof FIG. 3, transistor 270 is a bipolar transistor, and transistor 274 isa P-type MOSFET switch. The transistor 270 has a base which receives acontrol voltage from second voltage conversion circuit 40, a collectorconnected to a gate of transistor 274 and an emitter connected toground. The resistor 272 is connected between the gate of transistor 274and the drain supply voltage. The drain of transistor 274 is connectedto the drain supply voltage, and the source of transistor 274 isconnected via bias line 222 and output matching circuit 220 to the drainterminal D of transistor 20.

In operation, the control voltage supplied to the base of transistor 270is at ground in the absence of an RF input signal, and the gate oftransistor 274 is pulled to the drain supply voltage by resistor 272. Asa result, transistor 274 is off and the drain supply voltage is notapplied to the drain terminal D. When an RF input signal is received, acontrol voltage is produced on node 258 by the second voltage conversioncircuit 40, and transistor 270 is turned on. The gate of transistor 274is pulled to ground, and transistor 274 is turned on, thereby applyingthe drain supply voltage to the drain terminal D of transistor 20.

As discussed above in connection with FIG. 2, the gate bias voltage andthe drain supply voltage are sequenced such that the gate bias voltageis applied to the gate of transistor 20 before the drain supply voltageis applied to the drain terminal D of transistor 20. The sequencing canbe accomplished in the embodiment of FIG. 3 by appropriate choices ofthe components of the first voltage conversion circuit 30, the secondvoltage conversion circuit 40 and the switching circuit 50. Inparticular, the resistor 234 and the capacitor 236 establish a timeconstant of the first voltage conversion circuit 30, and the resistor254 and the capacitor 256 establish a time constant of the secondvoltage conversion circuit 40. The sequencing of the voltages may bebased on the difference between the time constants of the first voltageconversion circuit 30 and the second voltage conversion circuit 40.Thus, the values of the resistors and capacitors may be selected suchthat the time constant of first voltage conversion circuit 30 is lessthan the time constant of second voltage conversion circuit 40. In theembodiment of FIG. 3, it is assumed that switching circuit 50 has adelay which is short in comparison with the time constants of the firstvoltage conversion circuit 30 and the second voltage conversion circuit40. However, this is not a limitation and the switching circuit 50 canhave a selected delay.

A schematic diagram of an implementation of first voltage conversioncircuit 30 in accordance with embodiments is shown in FIG. 4. Likeelements in FIGS. 3 and 4 have the same reference numerals and theirdescriptions will not be repeated.

The implementation of FIG. 4 includes a four diode full-bridge rectifier410 rather than the single diode 232 of FIG. 3. The full-bridgerectifier 410 includes diodes 420, 422, 424 and 426 in a bridgeconfiguration. The input RF signal is connected through a DC blockingcapacitor 430 to the node between diodes 420 and 422, and the nodebetween diodes 424 and 426 is connected through a capacitor 432 toground. The node between diodes 422 and 426 is connected to ground. Thenode between diodes 420 and 424 (node 242) is connected to resistor 234and capacitor 236. A resistor 440 is connected between node 242 and gatevoltage regulator 240, and a capacitor 442 is connected in parallel withgate voltage regulator 240. The first voltage conversion circuit 30 ofFIG. 4 operates substantially as described above in connection with FIG.3, with improved performance provided at least in part by the use offull-bridge rectifier 410.

A schematic diagram of an implementation of second voltage conversioncircuit 40 and switching circuit 50 in accordance with embodiments isshown in FIG. 5. Like elements in FIGS. 3 and 5 have the same referencenumerals and their descriptions will not be repeated.

The second voltage conversion circuit 40 of FIG. 5 includes a four diodefull-bridge rectifier 510 in place of single diode 252 of FIG. 3. Thefull-bridge rectifier 510 includes diodes 520, 522, 524 and 526connected in a full-bridge configuration. The RF input is coupledthrough a DC blocking capacitor 530 to the node between diodes 520 and522. The node between diodes 524 and 526 is connected through acapacitor 532 to ground. The node between diodes 522 and 526 isconnected to ground. The node between diodes 522 and 526 (node 258) isconnected to resistor 254 and capacitor 256. Node 258 is connectedthrough a resistor 540 to the base of transistor 270, and resistors 542and 544 are connected between the base of transistor 270 and ground. Thecollector of transistor 270 is connected through a resistor 546 to thegate of transistor 274. A capacitor 550 is connected between thecollector of transistor 270 and ground. The circuit of FIG. 5 operatessubstantially as described above in connection with FIG. 3, withimproved performance provided at least in part by the use of full-bridgerectifier 510. A schematic diagram of transistor circuit 10 inaccordance with further embodiments is shown in FIG. 6. Like elements inFIGS. 1 and 6 have the same reference numerals, and their descriptionswill not be repeated.

In the embodiment of FIG. 6, the second voltage conversion circuit shownin FIG. 1 is replaced with a trigger circuit 610. The trigger circuit610 receives a trigger input from the first voltage conversion circuit30 and does not receive the RF input signal. The trigger input can betaken from node 242 (FIG. 3) of first voltage conversion circuit 30, forexample. The trigger input indicates the presence of an RF input asdetected by first voltage conversion circuit 30. The trigger circuit 610causes the control voltage to be applied to switching circuit 50 after adelay with respect to the gate bias voltage. The trigger circuit 610 mayinclude a delay circuit, such as an RC circuit, to delay application ofthe control voltage to switching circuit 50 with respect to theapplication of gate bias voltage to transistor 20. In other embodiments,the switching circuit 50 may include a delay circuit to delay theapplication of the supply voltage to the drain of transistor 20, and/orthe trigger input itself may be delayed by the first voltage conversioncircuit. The transistor circuit 10 of FIG. 6 has an advantage that asingle RF coupler can be utilized such that RF coupler 250 shown in FIG.3 is not required.

A variety of implementations are included within the disclosedtechnology. For example, the RF couplers 230 and 250 can be implementedas directional couplers in stripline or microstrip, transfomers,resistors, capacitors, etc. The diode rectifiers in first voltageconversion circuit 30 and in second voltage conversion circuit 40 may beimplemented as a single diode, as a two diode half-bridge rectifier oras a four diode full-bridge rectifier. In each case, the RF input signalis sampled, rectified and smoothed. The transistor 274 which switchesthe drain supply voltage can be any type of solid state switch, such asan N-type MOSFET, NPN or PNP bipolar transistors, GaN, GaAs switchingtransistors, or the like. As indicated above, the self-biasing disclosedherein can be applied to enhancement mode devices by appropriate changeof voltages. Further, the transistor circuit described herein can beimplemented as a discrete component, a chip-and-wire circuit on asubstrate inside the package of the transistor 20, or can bemonolithically fabricated on the same die as transistor 20.

The transistor circuit described herein may be utilized, for example, inan RF transmitter. However, this is not a limitation. Further, the RFinput signal, which may be in a range of kilohertz to tens of gigahertz,may be relatively narrow band. Again, this is not a limitation. Inaddition, the RF input signal may have a substantially constant powerlevel, except when turned off. Once again, this is not a limitationprovided that the RF input signal level is sufficient to generate a gatebias voltage and a control voltage.

Having thus described several aspects of several embodiments of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art. Such alterations, modifications, and improvements are intendedto be part of this disclosure, and are intended to be within the spiritand scope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

What is claimed is:
 1. A transistor circuit comprising: a transistorhaving a gate terminal and first and second conduction terminals; afirst circuit configured to convert an AC input signal of the transistorcircuit to a gate bias voltage and to apply the gate bias voltage to thegate terminal of the transistor, the first circuit comprising an RFcoupler, a rectifier, and a voltage regulator; a second circuitconfigured to convert the AC input signal of the transistor circuit to acontrol voltage; and a switching circuit configured to apply a firstvoltage to the first conduction terminal of the transistor in responseto the control voltage; wherein the first circuit, the second circuit,and the switching circuit are configured to apply the gate bias voltageto the gate terminal of the transistor before the first voltage isapplied to the first conduction terminal of the transistor. 2.(canceled)
 3. The transistor circuit as defined in claim 1, wherein thetransistor comprises a depletion mode transistor.
 4. The transistorcircuit as defined in claim 3, wherein the gate bias voltage is negativeand the first voltage is positive.
 5. (canceled)
 6. The transistorcircuit as defined in claim 1, wherein the rectifier comprises a dioderectifier.
 7. The transistor circuit as defined in claim 5, wherein therectifier comprises a half bridge rectifier.
 8. The transistor circuitas defined in claim 5, wherein the rectifier comprises a full bridgerectifier.
 9. The transistor circuit as defined in claim 5, wherein thevoltage regulator comprises a Zener diode.
 10. The transistor circuit asdefined in claim 5, wherein the RF coupler comprises a directionalcoupler.
 11. The transistor circuit as defined in claim 5, wherein thesecond circuit comprises an RF coupler and a rectifier.
 12. Thetransistor circuit as defined in claim 5, wherein the transistorcomprises a gallium nitride depletion mode power transistor.
 13. Amethod for operating a transistor having a gate terminal and first andsecond conduction terminals, comprising: converting an AC input signalto a gate bias voltage and applying the gate bias voltage to the gateterminal of the transistor using a first circuit comprising an RFcoupler, a rectifier, and a voltage regulator; converting the AC inputsignal to a control voltage; and after applying the gate bias voltage tothe gate terminal of the transistor, applying a first voltage to thefirst conduction terminal of the transistor in response to the controlvoltage.
 14. (canceled)
 15. The method as defined in claim 13, whereinthe transistor comprises a gallium nitride depletion mode powertransistor, wherein the gate bias voltage applied to the gate terminalof the transistor is negative and wherein the first voltage applied tothe first conduction terminal of the transistor is positive.
 16. Atransistor circuit comprising: a depletion mode RF power transistorhaving a gate terminal, a drain terminal and a source terminal; a firstcircuit configured to convert an input RF signal to a gate bias voltageand to apply the gate bias voltage to the gate terminal of thetransistor, the first circuit comprising an RF coupler, a rectifier, anda voltage regulator; a second circuit configured to convert the RF inputsignal to a control voltage; and a switching circuit configured to applya drain voltage to the drain terminal of the transistor in response tothe control voltage, wherein the first and second circuits and theswitching circuit are configured to apply the gate bias voltage to thegate terminal of the transistor before the drain voltage is applied tothe drain terminal of the transistor.
 17. The transistor circuit asdefined in claim 16, wherein the gate bias voltage is negative and thedrain voltage is positive.
 18. (canceled)
 19. The transistor circuit asdefined in claim 16, wherein the second circuit comprises an RF couplerand a rectifier.